If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

### Course: AP®︎/College Chemistry>Unit 6

Lesson 6: Hess's law

# Hess's law

Hess's law states that if a process can be expressed as the sum of two or more steps, the enthalpy change for the overall process is the sum of the ΔH values for each step. To use Hess's law, two principles must be understood: one, if an equation is reversed, the sign of the ΔH value is also reversed. Two, if an equation is multiplied by a coefficient, the ΔH value is multiplied by the same coefficient. Created by Jay.

## Want to join the conversation?

• At , how come the answer is not +226.8 kJ/molrxn? Isn't +1299.6 + (-787) + (-285.8)= 226.8 and not -226.8?
• Good eye, yeah they did make a math error. I checked around just to make sure and the reaction is endothermic so the enthalpy should be a positive value.
• Getting to a point where you can utilize Hess's Law just seems to convenient. How do chemists find equations that cancel out perfectly to get to that point?
• Chemists have recorded the enthalpy changes for most simple reactions using various calorimetry experimental techniques. Using Hess’ law we only need to experimentally determine the enthalpy of a relatively small number of the most simple reactions which then allows us to calculate the enthalpies of vast number of complex reactions.

For example here the actual reaction we want to know the enthalpy of is reacting solid carbon (graphite) with hydrogen gas to produce acetylene. So, we need a few simpler reactions which has graphite, hydrogen gas, and acetylene. And the three lower reactions involving those chemicals which we know the enthalpies of are about as simple as you can get in an experimental setting. All that is happening with those lower equations is that we’re reacting the desired chemicals with oxygen gas (essentially burning them) and observing the enthalpy change.

Hope that helps.
• Why exactly is reversing equations necessary?
(1 vote)
• We want the smaller reactions 1-3 to add in such a way that they make the overall reaction we began with. We need C2H2 as a product and the only way to get that from reaction 1 is to reverse it.
(1 vote)
• At why are we allowed to cancel out the products and reactants that appear on both sides of the chemical equation? On the one hand, it makes sense to cancel them, because if an amount of a chemical species was used up and then the same amount was also produced, you could argue that it's like, it was never there to begin with. However, on the other hand, couldn't you also argue that if that chemical species (that appears both as a reactant and a product) wasn't there, then the reaction might not happen?
(1 vote)
• Why are we reversing the step when the arrow of the reaction would indicate that the step only goes in one direction (In this case the Acetylene ) is being assembled instead of broken apart because the direction of said arrow stays the same, but this doesn't make any sense to me because it's not being communicated that the equation applies in both instances.
(1 vote)
• I'm not sure about whether the reaction is reversable or not, but I think the point here is that, logically, if one reverses an equation, its enthalpy change would be reversed as well.
(1 vote)